Place the potatoes and carrots into a large pot and cover with salted water. Bring to a boil over high heat, then reduce heat to medium-low, cover, and simmer until tender, about 15 minutes. Drain vegetables, then cover to keep warm.,Preheat an oven to 350 degrees F (175 degrees C). Spray a baking dish with cooking spray.,Melt 1/4 cup butter in a large skillet over medium heat. Stir in onion and garlic; cook and stir until the onion is softened, about 3 minutes.,Sprinkle in the flour and and reduce heat to medium-low. Continue to cook and stir for about 1 minute, whisking out any lumps.,Whisk in the milk until the mixture thickens, 5 to 7 minutes.,Flake the tuna, then stir it into the white sauce along with the potatoes, carrots, and peas. Season with salt and black pepper.,Transfer the tuna mixture to the prepared baking dish.,Mix 1/4 cup of melted butter with the crackers together in a bowl.,______,Bake in the preheated oven until the sauce is bubbly, about 30 minutes.